An adventure in Hertfordshire archaeology.

Tag Archives: magnetometer

Due to being rained off on our last day, a small team of us decided to go out and finish off some things on Bank Holiday Monday. Many thanks to Pauline, Judith, Ruth, Dave and Jim for turning out to do “just one more grid.” I think it must be a geophysicists ailment that we always would like to be able to just a little bit more…

The mag team completed an impressive ten grids including two awkward partials. Figure 1 shows the entire survey at the end of the 2018 season.

Figure 1: the mag survey after day 19.

The team have managed to add 19 ha to the survey in the last month. Figure 2 shows the southern area that we have been surveying this week. (This field is, confusingly, called “Prae Wood”.)

Figure 2: the southern area (Prae Wood) after day 19.

The team have picked-up an area of intense ferrous noise. This looks like a small historic period site. We will have to check out some old maps to see if we can work out what that might be. The one hiccup in a brilliant last day of work is a single line of data where the sensor froze. It is very annoying and I’ll have to find some way of fudging that until next summer!

The Earth Resistance survey had one last little block left to make the plot look all neat and tidy. Many thanks to Pauline and Judith for helping me fill that in (Figures 3 and 4)!

The data collected shows some faint indications of buildings in that corner (Figure 5).

Figure 5: the northern area of the res survey. The NW corner was completed on day 19.

Although my trick of spreading the remote probes wide apart has worked on the whole, this year there is a bit of an edge. This is because we started with a block in the SW corner, worked eastwards, and then when we had got to the corner, worked back along the hedge line westwards. Between when we started this block and yesterday we have had in excess of 100mm of rain (or about 4 inches in old money) so it isn’t surprising this shows in the results.

We have now cleared away all the pegs and flags, packed-up the machines and left Gorhambury for another year. It is a beautiful place to work and we are very grateful to Lord and Lady Verulam and their family for allowing us to extend the survey, to those who work the estate and put up with us getting in the way, and to the estate managers, especially Stuart Gray. Thanks to the Institute of Archaeology, UCL, for lending us the dGPS and the res meter, and SEAHA for the loan of the GPR. I hope everyone involved thinks the results are worth all the effort. Most of all I would also like to thank all the volunteers who came this year, whether you only managed a day or two, or you came for the whole season. You are what makes this project so much fun!

We had a large team today and as a result we managed ten mag grids, two and a bit GPR grids and seven earth resistance grids. Good job everybody!

First to the mag. The team extended their survey in the field to the south of Mobbs Hole. Figure 1 shows the overall survey and Figure 2 zooms in on this field.

Figure 1: the 2018 magnetometry survey.

Figure 2: the survey in the field to the south after day 2.

I have annotated Figure 2. The red arrows indicate the line of the ditch of the Fosse. It is salutary to note that a feature as big as the Fosse barely shows in the mag data. Clearly the upper fills of the ditch are largely the same soil as the surrounding topsoil. We can normally see pits and ditches on archaeological sites because they are filled with more organic, and thus more magnetic, soils, the result of nearby human occupation. The green arrow shows a “blob” of higher magnetic readings. The rather diffuse edges to this feature make me suspicious that this might be a “tree throw”, i.e., where a tree has blown down. The yellow arrow marks two strongly magnetic parallel lines. At first I thought these might be something metallic but checking their actual values shows they vary from -10 to +29 nT. Certainly strong, but unlikely to be metal. The blue indicates something which is definitely metal; it has values of -1543 to +680nT! The dark pink arrows indicate a faint line, possibly an old fence line.

We have now covered 6.3ha. For a resistance survey at 0.5m spacing between readings, that is pretty impressive. Res has always been a poor third to mag and GPR in this survey. We didn’t get started until a year after the other techniques when UCL purchased a new RM85, and we have had problems with weather. Hopefully we can fill in the top corner on Saturday.

Figure 4 shows a detail of the area completed this year.

Figure 4: the northern area completed so far this season.

The street shows very clearly in Figure 4 running SW-NE, and slightly more faintly we can see the buildings either side. One problem to tackle in processing data is that very high areas, like the road, can make the more subtle stuff harder to see. If we “clip” the image to bring-up the details of the buildings, the road area becomes one big black blob! One way to get around this is to use a high-pass filter. Figure 5 shows the same area with the high-pass filter applied.

Figure 5: the 2018 survey area after the application of a high-pass filter.

As you can see, the buildings show much more clearly but the road much less so. Especially with resistance data, it is worth looking at several versions of the data processing to get the most detail from the survey.

The GPR crew finished off the grid from yesterday and did another 40x80m block. Figures 6 and 7 are the time slices from the two days.

Figure 6: time slices from day 16 of the GPR survey.

Figure 7: time slices from day 17 of the GPR survey.

As you can easy see, we have some sweet buildings showing. Figure 8 is a rough composite of the sites in this area.

Figure 8: composite of slices in the area of the day 16 and 17 survey blocks.

I need to do some cleaning-up of the various blocks as they were processed at different times and with different software packages, but in general you can see the mass of buildings crowding along this section of road. Very nice!

Signing off now so we can go and start day 18. This may be our last day as the weather forecast for Sunday is dire…

Week three started well with all three machines collecting data. The Earth Resistance survey was the poor cousin as regards person-power but Ellen and I, helped by Rhian, completed three grid squares after lunch.

Figure 1: Ellen and an earth resistance meter.

The grids are over the fascinating burnt building seen in the mag. Figure 2 shows the mag data in this area.

Figure 2: the mag data in the area of the res survey.

The black line snaking across Figure 2 from top-left to bottom-right is the aqueduct. The very bright black-and-white area in the NW corner of that figure is probably a burnt building which was never replaced in stone. Figure 3 shows the Earth Resistance data.

Figure 2: the Earth Resistance data.

Figure 3 is a crude composite of the data collected in 2016, 2017 and 2018. The three squares at the north edge are this year’s grids. We have clearly picked-up a long wall running NW-SE, and some square areas of higher resistance (?floors, maybe). This survey makes an interesting comparison to the GPR in this area (Figure 4).

Figure 4: the GPR survey in the same area as Figures 1 and 2.

There is a lot of work to do tracing off walls and features from the three surveys.

The GPR crew completed another 80x40m block, although the slope was quite a challenge. Figure 5 shows the time slices.

Figure 5: day 11 GPR time slices.

Slices 4 and 5 (top-right and middle left) seem the most interesting. No stunningly clear buildings but lots going on. Figures 6 and 7 compare the fifth slice with the mag data.

Figure 6: Fifth time slice from day 11 (indicated by the purple line).

Figure 7: the mag data in the area of the day 11 GPR data (shown by the purple box).

Notice how the square of higher mag response shown in darker tones towards the bottom of the purple rectangle are an area of light “low reflections” in the GPR data. It is possibly something like an earth floor? Off the west corner of that square in the mag data is a lighter coloured line running to the SW which is matched by a black line of high reflections in the GPR data. That is clearly a wall, probably made of flint. The very narrow section of the aqueduct which runs east-west across the plot shows very clearly in the GPR data whereas the broader sections do not. Something odd happens with the aqueduct at the eastern edge of the GPR plot. A great deal more to tease out.

One problem we have had this year is the sheep. In general they keep away from us. The main issue is that some of them think the flags are tasty… (Figure 8).

Figure 8: Tasty! A nibbled flag in Mobbs Hole with the mag crew in the background.

The mag crew consisting of Jim West, Peter Alley and Dave Minty had three annoying partials to do before marching eastwards across the field. I’m afraid I have not finished processing those annoying squares but I have added in the complete ones to Figure 9 so you can see progress.

Figure 9: mag data in Mobbs Hole after day 11.

Unfortunately, today was a bust as it rained 8.5mm. The forecast for tomorrow is looking good though.

Despite the unpromising weather forecast (and the unpromising weather at about 8.30am), we managed a whole day of survey including mag, GPR and a little bit of earth resistance. There was some drizzle around about lunchtime, but other than that is was an OK day, if a little windy at times.

The GPR team were working on “sawtooth” edges to the theatre field. As we have a couple of days off, I will post the results of their efforts tomorrow.

The mag team completed twelve grids, including two partials and despite one case of “sensor freeze”; a super effort (Figure 1).

Figure 1: the mag crew, consisting of Ruth Halliwell, Jim West and Dave Minty, operate the Foerster in Mobbs Hole.

Figure 2 shows all the areas surveyed so far at Verulamium.

Figure 2: all the mag survey so far.

This really shows that the strength of the Foerster cart system is when one has large open fields. So far, according to TerraSurveyor, we have completed 9.8ha of Mobbs Hole over ten days, but remember we only had part of an afternoon on day 1 and lost parts of two days this week to rain. Compare that to surveying in Verulamium Park with all its trees, hedge lines, park benches and so forth. There we surveyed just under 30ha in 45 days.

Figure 3 zooms into the area surveyed over the last few days.

Figure 3: mag survey after day 10.

So far the results are curious. There are linear features associated with the line of the Fosse, plough scars running NW–SE down the slope, a few large “blobby” anomalies (pits, maybe?) and some bits of old iron. Very little which could be interpreted as structures has been found. Why build an enormous ditch and bank around nothing? Ploughing may have removed some superficial features, but there is very little that could be seen as pits or ditches either. The Fosse remains a mystery.

Figure 4 shows where the Fosse enters the wood. The dip in the fence marks the ditch. Once inside the wood the Fosse does a sharp left turn and heads southwards.

Figure 4: the dip in the line of the fence shows where the Fosse enters the woodland.

Tomorrow I will report on the earth resistance and GPR surveys in the theatre field.

Many thanks to everyone who has helped this season. We are already half way through. Doesn’t time fly?

Not much sign of corn, but the wind was blowing over the parched grass. It was a welcome relief to be working in cooler conditions, although the weather was still beautiful.

Figure 1: St Albans Abbey.

The mag team completed an excellent eight grids in Mobbs Hole (Fig. 2).

Figure 2: the mag survey after day 6.

As before I have overlain the survey on the 2006 imagery in Google Earth which shows the Fosse most clearly. The hints of a line along the inner edge. Maybe this is the remains of a palisade trench? Disappointingly little otherwise. One thing to note is that the “noise” from random ferrous trash is more prevalent to the south of the old fence line than to the north. I wonder if the NW corner of this field was pasture previously?

The GPR crew completed their two grids. Figure 3 shows some time slices.

Figure 3: Day 6 GPR results.

Not a great deal showing apart from in slice 4 (in the top-right corner of Figure 3) which clearly shows the aqueduct. This is about as clear as I have ever seen it in GPR data. Awkwardly, the direction of the transects is close to the direction of the aqueduct. Figure 4 shows the slice in context with the others we have surveyed this year.

Figure 4: GPR day 6, slice 4.

The edges do not match because I keep playing with the settings in the software. One day, I’ll slowly process the whole lot so that we get a nice final result. One day (more like several months…).

Barney and Becca came and helped with the GPR in the morning. After lunch, we blew the dust off our Bartington and did a couple of squares. I rather liked this image of Barney.

Figure 5: Barney and a Bartington.

The survey moves on and we cover more ground. Many thanks to everyone who comes and helps expand the area we have covered. Luckily, most of the area you can see in the last inage has already been done!

At the request of Andrew Reynolds and Stuart Brookes, medieval archaeologists at the Institute of Archaeology, UCL, members of CAGG headed out on tour to Wiltshire. The site we were asked to survey was the Iron Age hillfort of Chisbury, near Great Bedwyn, Wiltshire. One may ask why Chisbury is of interest to early medievalists? An early 10th century document called The Burghal Hidagerecords a site called Cissanbyrig (Baker and Brookes 2013, p. 228) which may be Chisbury. The Burghal Hidage records the defended settlements (burhs) of the Kingdom of Wessex set-up after the defeat of the Vikings in the late 9th century. The Historic England Scheduled Monument listing notes that:

Although no formal excavations have been carried out within the hillfort, observation of 20th century disturbances has produced evidence of urns, bronze swords and of storage pits containing Late Iron Age and Romano- British pottery.

The site also has a well preserved 13th century chapel under HE Guardianship.

Fig. 1: the chapel at Chisbury.

The interior is quite plain but has some interesting details.

Fig. 2: the interior of the chapel at Chisbury.

One of those details is a surviving consecration cross painted on the back wall. Apparently these were painted on the wall when the church was dedicated.

Fig. 3: The consecration cross.

Peter Alley’s high-level photograph from the UAV shows how the chapel lies right across the defenses of the hillfort. Maybe this position was significant in somehow slighting the earthworks?

Fig. 4: high level photo showing the location of the chapel.

The defenses are very well preserved around most of the circuit but they are covered in trees so hard to see and photograph. From the satellite image the site is an extended oval of trees.

Fig. 5: Chisbury from Google Earth.

Jim West took a good image showing part of the defences to the west.

Fig. 6: Chisbury defences from the west (photo: Jim West).

The weather was wonderful and we all got a bit sun burnt. The bluebells and primroses were out in force.

Fig. 7: bluebells on the defences.

The theme of the week was, however, definitely “horses”!

Fig. 8: Nosey horsey.

Unfortunately, horses are quite magnetic due to their ferrous footwear.

Fig. 9: Horseshoe alert! (Photo: Mike Smith).

Those horses are also a bit careless with their shoes…

Fig. 10: Missing footwear.

The plan was to complete as much of the inside of the fort as we were able with the magnetometer, and to do some selected areas with the GPR and the Earth Resistance meter. Right from the beginning we were beset with problems. We arrived at lunchtime on the Thursday, and managed to complete quite a few squares in the first afternoon, but the odometer started to over-run, eventually by four or five meters. I swapped a few emails with Pat Johnson from Foerster, and the next day we managed to cure the problem. A couple of days later, one of the pins in the “spider” — the cable that joins the sensors to the control box — snapped so we were down to three sensors. On the last full day, the odometer started slipping again… We did manage to survey the whole available area of the fort, but only just and without much time to try the other methods. Fig. 11 shows the overall survey.

Fig. 11: magnetometry survey.

Although some of the major features can be seen at this scale, I have created two images with the north and south parts of the survey and some annotations. (You might like to look at these downloaded and at full size.)

Fig. 12: the mag survey of the northern part of the site.

In Fig. 12 we can see a series of parallel linear features which have been annotated in cyan. These look like field drains to me. Very faintly, however, there are some circular features. These may well be the “drip gulleys” of Iron Age circular round houses. I have marked some of the possible ones in red. The problem with these is that the more one stares, the more one invents! I am sure you can spot a few more possibles if you look long enough. There is a great deal of ferrous noise, especially around the edges from fences, gates and water tanks, but also in the field from old nuts and bolts, horse shoes and the like. Looking carefully and the little blobs and measuring the minimum and maximum values in nanoteslas (the unit of magnetism), one can start differentiating between bits of old iron and possible pits. I have marked a small number of the possible pits with green arrows.

Fig. 13: the mag survey of the southern part of the site.

In Fig. 13 the red line is the pipe which joins the main water tank in the middle of the field. I have marked just one piece of ferrous rubbish with a red arrow, there are lots more. The cyan lines mark the possible field drains. The dark blue line is a negative-magnetism feature which runs from the edge of the water tank to the pond. This can also be seen in the GPR data (below). I am guessing this is some sort of drainage / outflow from the water tank to the pond. There are some areas with such high ferrous noise it is impossible to see anything, for example the north end of the eastern field. There are, however, quite a few pits once more, and I have marked just a small selection. What is curious, however, is how much of this area seems devoid of any features at all.

Although the mag results are not exciting in the sense of being able to clearly see a building, as we often do at Verulamium, there is quite a bit of detailed information buried in the data.

We managed just a couple of days of GPR survey thanks to some local help.

Fig. 14: the GPR survey underway. (Photo: Mike Smith.)

I used the GPR Process and Surfer programs and created 3ns time slices.

Fig. 15: GPR time slice 3 from 10.5 to 13.5ns.

Fig. 16: GPR time slice 4 from 13.5 to 16.5ns.

Fig. 17: GPR time slice 5 from 16.5 to 19.5ns.

Fig. 18: GPR time slice 6 from 19.5 to 22.5ns.

Fig. 19: GPR time slice 7 from 22.5 to 25.5ns.

Fig. 20: GPR time slice 8 from 25.5 to 28.5ns.

To be frank, not a great deal shows. The last time slice shows the suggested pipe from the tank to the pond. The southern area has more high reflections, especially in slice 5 (Fig. 17) which one could try to make into buildings, but I find myself suspicious that these features are close to the water tank and they may be something to do with the tank’s construction.

Giving the billing this site has, the results are not all that stunning. There are, however, features which would be worth investigating further, and hopefully we will get to “ground truth” some of these features as part of Andrew Reynolds and Stuart Brookes’ wider project.

Many thanks to those CAGG members who came all this way to do the survey: Ellen Shlasko, Ruth Halliwell, Peter Alley, Jim West, Nigel Harper-Scott and Mike Smith. Many thanks too to the members of the local group who came to help: Shaun Wilson, James Kay and Lynn Amadio. Lastly, but certainly not least, thanks to the stud and the farm for allowing us to play on their land.